Princess Diana did something that upset her elder son, Prince William.

In the documentary “Prince William at Thirty,” the Princess of Wales shocked the world when she candidly shared her troubled marriage with Prince Charles to Martin Bashir for the BBC’s “Panorama.” Princess Diana bravely talked about her mental illness and infidelity on national television.

The Duke of Cambridge had always supported his mother as he witnessed how his parents’ disagreements. However, he felt that Prince Diana “had gone too far” when she did the interview.

“For him, the idea of playing out one’s troubles and wearing one’s heart on one’s sleeve and bringing humiliation onto the Royal Family – well, that was a big no-no,” royal biographer Katie Nicholl said.

“He refused to speak to his mother for some days after that and bare in mind how close they were, this was absolutely devastating for Diana.”

Former BBC royal correspondent Jennie Bond added that the second-in-line to the throne was so upset because he couldn’t believe what his mom did.

“William was desperately upset. Couldn’t believe she’d laid her soul so bare,” Bond added.

Princess Diana’s royal press spokesman Dickie Arbiter also didn’t agree with what she did. He even considered it the “stupidest thing she has ever done.” Princess Diana had been known for breaking royal protocol, but her infamous “Panorama” interview was the last straw for the royal family.

Royal author Penny Junor felt that Princess Diana’s interview was her way to “publicly execute” the heir apparent. The biographer said that the People’s Princess did a “superb performance” during the interview that was worthy of an Oscar. She added that the princess’ revelations were a big secret and no one had an idea about it.

Junor continued saying that by doing so, Princess Diana manipulated Bashir the way she did with her biographer, Andrew Morton. Princess Diana turned her private battle with Prince Charles into a public execution without considering her children.

Meanwhile, Prince William and Prince Harry were shocked and confused about the public’s reaction to their mother’s death. The Duke of Cambridge admitted that there was a time he felt a bit protective because he couldn’t understand why many people grieve for his mother even if they didn’t know Princess Diana personally. However, when he learned what his mom gave to the world, he understood the people’s reaction.

Princess Diana, Prince William, Prince Harry
Pictured: Princess Diana, Prince Harry, Prince William gather for the commemorations of VJ Day, 19 August 1995, in London. Getty Images/Johnny Eggitt/AFP